Importin-Beta and Ran Regulate the Passive Permeability Barrier in the Nuclear Pore Complex

The nuclear pore complex (NPC) is a large, multi-protein assembly that mediates the selective transport of molecules between the cytoplasm and nucleus in eukaryotic cells. While the proteins involved in the transport pathway have largely been identified, the physical mechanism by which this complex can support both efficient and selective molecular transport remains unclear. Using fluorescence microscopy and super-resolution imaging techniques, we have examined how nuclear transport receptors influence the permeability properties of the NPC for both active and passive transport processes. We find that importin-β binding to the nucleoporin Nup153 significantly slows passive transport through the NPC; however, Ran in its GTP-bound form reverses this effect. FRAP studies reveal that RanGTP weakens the binding interaction of importin-β to the NPC. Furthermore, STORM imaging of individual importin-β localizations inside the NPC show that RanGTP dramatically changes the importin-β distribution within the channel. These results suggest that importin-β in conjunction with Nup153 is an integral component of the NPC permeability barrier which is regulated by RanGTP.